This invention is directed to a perishable good integrity indicator, and more particularly to an apparatus for accurately determining the shelf life of a perishable good.
Perishable goods such as foods, medications and the like lose their potency and/or their nutritional value over time when exposed to temperature. The time period during which perishable goods maintain their nutritional value and/or potency is known as the shelf life. Once the shelf life has expired, the perishable good may be of no further value and in many cases, may actually be harmful if utilized as in the case of spoiled food.
A perishable good will have a given shelf life at a given temperature. As the temperature increases, the shelf life normally decreases, and the shelf life normally decreases at some exponential rate. The shelf life and integrity of different goods varies as a function of optimum storage temperature, the length of time the product will last at the optimum temperature, the deterioration curve of the product due to increased heat, and the affect on shelf life and product integrity due to decreased heat or severe cold.
In the prior art, shelf life is determined in advance for each individual product based upon empirical data and based upon assumed conditions during storage. An expiration date based upon this estimated shelf life is then stamped on the product and as a rule, these perishable goods are not sold after the expiration date. This system has been satisfactory, however it suffers from the disadvantage that the shelf life determination does not accurately reflect the actual conditions to which the perishable good is subjected, such as the temperature at which the good is stored in individual stores. Accordingly, products which have been stored in excessive hot or cold conditions are maintained on shelves well beyond the expiration of their shelf life and products which are optimally stored are often removed in accordance with the stamped date prior to the expiration of their actual shelf life.
Shelf life is a function of time and temperature. Devices for measuring time and temperature deviation are known in the art and use oscillation circuits which vary output frequency with temperature deviation. Analog compensation circuits are utilized to increase or decrease the amount of frequency variation at a given temperature range and the frequency deviation is measured as an estimated temperature deviation. These electronic temperature measuring devices are satisfactory, however they suffer from the disadvantage that it is prohibitively expensive to accurately build oscillators with temperature consistent characteristics utilizing standard manufacturing processes. Accordingly, a frequency calibration circuit must be incorporated to compensate for manufacturing variations. This calibration requires time and special equipment which increases the cost of the product by requiring that the circuit include a calibration interface and possibly significant non-volatile memory. Additionally, the temperature variation of the oscillator with respect to frequency is subject to numerous factors making it difficult to achieve the correct temperature to frequency correlation curve for a desired time temperature application. Additionally, different perishable goods have different shelf life parameters making the prior art oscillator based time temperature indicators impractical and/or inaccurate.
Accordingly, a time temperature measuring device capable of indicating a product shelf life which overcomes the shortcomings of the prior art is desired.